Transmission Electron Microscopy Study of Conversion of Smectite to Illite in Mudstones of the Nankai trough: Contrast with Coeval Bentonites

Harue Masuda1, Donald R. Peacor2 and Hailiang Dong2, †
1 Department of Geosciences, Osaka City University, Sumiyoshi, Osaka 558-8585 Japan
2 Department of Geological Sciences, The University of Michigan, Ann Arbor, Michigan 48109, USA
Present address: Department of Geology, Miami University, Oxford, Ohio 45056, USA. E-mail of corresponding author: harue@sci.osaka-cu.ac.jp

Abstract: Clay minerals in shales from cores at Site 808, Nankai Trough, have been studied using X-ray diffraction (XRD), scanning transmission electron microscopy (STEM), and analytical electron microscopy (AEM) to compare the rates and mechanisms of illitization with those of coeval bentonites, which were described previously. Authigenic K-rich smectite having a high Fe content (∼7 wt. %) was observed to form directly as an alteration product of volcanic glass at a depth of ∼500 meters below seafloor (mbsf) with no intermediate precursor. Smectite is then largely replaced by Reichweite, R, (R = 1) illite-smectite (I-S) and minor illite and chlorite over depths from ∼550 to ∼700 mbsf. No further mineralogical changes occur to the maximum depth cored, ∼1300 m. Most smectite and I-S in shales are derived from alteration of glass, rather than being detrital, as is usually assumed. Discrete layer sequences of smectite, I-S, or illite coexist, indicating discontinuities of the transformation from smectite to (R = 1) I-S to illite. Authigenic Fe-rich chlorite forms concomitantly with I-S and illite, with the source of Fe from reactant smectite.

Smectite forms from glass with an intermediate precursor in coeval bentonites at approximately the same depth as in shales, but the smectite remains largely unchanged, with the exception of exchange of interlayer cations (K → Na → Ca) in response to formation of zeolites, to the bottom of the core. Differences in rates of illitization reflect the metastability of the clays. Temperature, structure-state, and composition of reactant smectite are ruled out as determining factors that increase reaction rates here, whereas differences in water/rock ratio (porosity/permeability), Si and K activities, and organic acid content are likely candidates.

Key Words: Bentonites • Diagenesis • Illite • Marine Sediments • Mixed-Layered I-S • Nankai Trough • Shales

Clays and Clay Minerals; April 2001 v. 49; no. 2; p. 109-118; DOI: 10.1346/CCMN.2001.0490201
© 2001, The Clay Minerals Society
Clay Minerals Society (www.clays.org)